Various optics applications, including for example image recording and image acquisition, optimally require high brightness illumination. Brightness for a luminous source is defined as a luminance intensity (power) per unit area per unit solid angle.
It is well known that because of the law of conservation of brightness, the brightness of a source determines a theoretical maximum brightness of illumination that can be delivered to a surface. If a location on a surface is to be illuminated with illumination having a given optical numerical aperture (NA) using all of light available from a given source of illumination, then no more light can be provided to that location merely by simplistically combining illumination from additional emitters.
There are however several special cases in which illumination from several emitters can be combined in order to increase apparent brightness. These include, for example, wavelength multiplexing of narrow spectrum sources and polarization combination. Such methods require that the emitters emit light having a narrow spectral content or emit light that is polarized. The illumination that results from such multiplexing has increased spectral content or becomes non-polarized.
A typical application for recording information on a photosensitive surface is the direct imaging of electrical circuit patterns on a photoresist clad printed circuit board substrate. Widely used commercially available photoresist products are sensitive to UV light (about 370 nm) and conventional direct imaging exposure systems employ modulated UV lasers. Considerable research and development work is being applied to the development of inexpensive high brightness semiconductor lasers, however these lasers still do not emit light in a spectral bandwidth able to expose widely used commercially available photoresist products. Conversely, light emitting diodes having a spectral bandwidth that is able to expose commercially available photoresist products are now being introduced to the marketplace. These new light emitting diodes however typically do not provide illumination having sufficient brightness to be used in commercially viable exposure systems.
A light source for providing high power incoherent light with a laser array is described in U.S. published patent application 2002/126479 A1.